back to article Supercapacitors have the power to save you from data loss

As solid state drives (SSDs) become a critical part of today's storage, it is becomes increasingly important to learn about the supercapacitors that help prevent data loss. The presence – and type – of supercapacitors in SSDs should be as important a consideration as choosing between MLC, eMLC and SLC-based drives. …

  1. leon clarke

    Very interesting

    But any chance of a noddy version of this article - that actually mentions particular models of SSD instead of particular capacitor chemistry options.

    1. A Non e-mouse Silver badge

      Re: Very interesting

      Or at least a way for us to tell if our SSDs have supercapacitors onboard.

      1. BlartVersenwaldIII

        Re: Very interesting

        As a rule of thumb, if a drive doesn't advertise as having power-loss protection then it doesn't have it. The only consumer-level ones I'm aware of with capacitors are the crucial drives from the M500, M550 and MX100 ranges and some of the Intel ones (although it's been a while since I've checked). Most business/enterprise-oriented drives have them as well. The degree to which you need power protection varies a great deal depending on how the SSD is architected - some controllers write straight to flash (therefore need no DRAM cache to flush) yet still use RAM for the cell map.

        In response to another post, UPSs don't address all possible modes of failures - for instance accidentally pulling the power cable out from a running drive (or array enclosure), over-eager sleep/hibernate yanking power from the drive before it's done writing or even just the power supply going foom, all of which are quite common in desktop scenarios. This was one of the sure-fire ways of bricking your OCZ drive with their horribly defective sandforce firmware.

  2. Danny 14

    Good old UPS with a reserve and shutdown is also a good idea. Then again, some SANS wont accept UPS shutdown signals....

    1. DaLo

      "Good old UPS with a reserve and shutdown is also a good idea"

      As already talked about in the article.

      "some SANS wont accept UPS shutdown signals"

      Really, not simply a NAS...? And you would use this as a 'SAN', to protect real data?

      1. Captain Scarlet Silver badge
        Meh

        My opinion is I would not want my SAN(s) to shutdown at the same time as my servers, in theory if everything is done writing would you worry about losing power?

        If the SAN shuts down at the same time what about that crusty old Windows Server with a hideous program that takes 5 minutes to close that everyone has somewhere hidden from view?

        1. DaLo

          A UPS shutdown signal doesn't mean turn yourself off now!

          You can send the signal to start a countdown timer or, on most UPS', start warning that it's on battery and keep warning until the supply is critical - even just run a completely custom script that will check critical servers to see if they are still connected before shutting down.

          If you've got a crusty old Windows server that takes 5 minutes to shutdown then I expect the data isn't really that critical on it?

  3. FartingHippo
    Mushroom

    Kaboom?

    I'd like to nominate "a small problem with some minor exploding" for the British Understatement in Journalism Award.

    1. Trevor_Pott Gold badge

      Re: Kaboom?

      Ooooh, Canadians qualify?

      1. FartingHippo
        Holmes

        Re: Kaboom?

        "Canadians Qualify?"

        Of course! Generally good eggs, Canadians (apart from the wretched Québécois, obviously).

        1. Trevor_Pott Gold badge

          Re: Kaboom?

          Q_Q I are French and Dutch, with a strong ancestry from Quebec and France...

          1. FartingHippo
            Unhappy

            Re: Kaboom?

            Commiserations. Still, at least you can supplement your cheese with a smoke and a pancake.

    2. Adam 1

      Re: Kaboom?

      Personally, I prefer "Supercapacitors are like batteries, but more awesome"

      1. Dave 126 Silver badge

        Re: Kaboom?

        "Oh, meltdown. It’s one of those annoying buzzwords. We prefer to call it an unrequested fission surplus."

      2. Fred Flintstone Gold badge

        Re: Kaboom?

        Personally, I prefer "Supercapacitors are like batteries, but more awesome"

        Seconded :)

  4. Warm Braw

    Back in the day...

    I seem to remember that the angular momentum in those washing-machine sized drives was used to generate the power to complete the pending writes and withdraw the heads in the event of the electricity going off.

    Mind you, if the bearings seized, you'd have more than "a small problem with some minor exploding".

    1. John Smith 19 Gold badge
      Unhappy

      Re: Back in the day...

      "I seem to remember that the angular momentum in those washing-machine sized drives was used to generate the power to complete the pending writes and withdraw the heads in the event of the electricity going off."

      We used to cal them "Twin tubs." With an aircon failure in the machine room in Summer they could cook you pretty well.

      That sounds like a UL as the exchangable ones packed a fair amount of energy that generation of write heads would have needed quite a bit to flush any caches.

    2. Frederic Bloggs

      Re: Back in the day...

      The crucial thing that the reservoir of power gave was the ability to guarantee that the current block would be written. This gave rise to some of the earliest logging filesystems, which resulted in a quantum leap in reliability in the (frequent) power problems we used to have. Mind you a huge 100KW motor-dyno-generator helped enormously come 8:30 in the morning when all the heavy machinery on our industrial estate started up more or less at once. Nothing like an huge flywheel to smooth out power spikes! (Happy times).

      It's nice to storage principles being dragged kicking and screaming into the 1960s (again).

  5. Anonymous Coward
    Anonymous Coward

    Correction

    "Wet supercapacitors are usually based on aluminium.

    Wet supercapacitors based on graphene and carbon nanotube promise to change this all over again, but neither are available in volume."

    Not entirely true. The aluminium in what the author calls wet supercapacitors is merely a current collector. The electrode material is almost always some flavour of ultra porous carbon. The lifetime issue is due to the difficulty in packaging this material to keep the good stuff in and the bad stuff (mainly atmospheric moisture) out.

    1. Frederic Bloggs

      Re: Correction

      The "wet" capacitors are essentially electrolytic capacitors where the (very thin) "plates" of the capacitor are held apart by (again very thin) slightly damp "insulator" material. This dampness will, over time, evaporate out of the device through the weeny vent provided. This is why these capacitors have a finite life.

      Obviously the hotter they get, the faster the electrolytic dampness evaporates. In the limit it will boil and, generally, an engineered weak spot in the cap will blow and, again generally, cause the case to fly away from the motherboard with a characteristic bang, clatter (as the cap body hits something) and nasty smell. Sometimes the cap develops an internal fault, this makes the boiling happen too fast for the safety valve; this is when the cap actually explodes.

      But don't worry if you keep your computer case shut, you'll be fine.

  6. Anonymous Coward
    Anonymous Coward

    "That means that wet supercapacitors are soldered to SSDs by hand,"

    That seems dubious - I'm not an expert on production line electronics but if it can be done by hand then surely you can automate it - just replicate the manual soldering process? However, even at higher temperature surely a heat shunt or even just a cooled probe which could drop onto the component to protect it be used. I can't believe that any mass production of SSDs has a line of hand soldering workers who can create consistently reliable results.

    1. Trevor_Pott Gold badge

      My understanding is that the typical industry practice of reflow soldering basically can't work here. That means you either need an expensive (and apparently not very accurate) robot, or humans. I am sure some companies automated. Others apparently did choose the human route, as it was the only component that couldn't be reflow soldered. Never ask how the sausage is made. You will never like the answer.

    2. Neil Barnes Silver badge

      Er, no, automating 'soldering iron' soldering is not as far as I know practicable. Flow soldering requires the entire circuit board and all its components to be raised in temperature until the solder flows - said solder being placed on the board in precise amounts as required, and the temperature profile is set so that everything reaches the proper temperatures at the same time, give or take.

      Doing it with a soldering iron means that the component is subject only to the heat transferred from the leads (or tabs, or whatever) while the joint is being made - a few seconds rather than a few minutes, and with much less energy transfer.

      <edit - posted as you did yours, Trevor; it's a response to the first post, not yours!>

      1. DropBear

        Er, no, automating 'soldering iron' soldering is not as far as I know practicable

        As far as I know, components get soldered by hand not because the soldering operation requires human precision, but because either they have problematic through-hole pins and are not easily placed by machines, or because they have large-ish heat-sinking parts that result in inconsistent soldering if relying on reflow alone. To be honest, I would see no problem with a robot placing a SMT cap on the already-reflowed PCB then another robot coming in with a small hot-air nozzle (doesn't even need to be particularly precise) to quickly solder it in. Then again, I never purported to work on an assembly line, so what do I know...

      2. John Tserkezis

        "Doing it with a soldering iron means that the component is subject only to the heat transferred from the leads (or tabs, or whatever) while the joint is being made - a few seconds rather than a few minutes, and with much less energy transfer."

        While you're right, that may not be the reason at all.

        If you're using a supercap that's going contain enough charge to not only run the on-board CPU, read and manage the DRAM, and write out the flash, it'll have to be a bigger one.

        Larger components, surface mount or not, usually cannnot be used while wave soldering the board, simply because it's too high, meaning the wave flow won't reach the solderable bits. The solder "fountain" is simply not deep enough.

        One way to cheat, is to use a supercap compatible with through-hole tech, and place it AFTER you do the supercap component side (and wave solder the other side, doing the cap along with everything else).

        In some cases, that's not possible, if the designers have decided to go for a single-sided approach, in which case, manual soldering is left.

        1. Dave 126 Silver badge

          If the SSD manufacturers are expecting a more heat-tolerant capacitor chemistry to be available soon, they might not consider it worth investing in developing automated soldering techniques for the current generation of capacitors.

          That's just a thought, I really don't know.

    3. Anonymous Coward
      Happy

      You can if your personnel are either former/current jewel-smiths and/or NASA 2M (Micro-Miniature) certified. Both types are machines when it comes to soldering. My mother's the former while I created and stood up a computer repair facility leveraging the latter. [I used to be certified to do that work, physically impossible now, but I don't care for it all! My wire-wraps though still break torsion-meters. So you can guess how I build these days :-).] When you compare the cost of replacing a board vice replacing the board, you can save a ton of money. And I mean a literal ton.

  7. Tom 7

    I've got one in an 8 year old FM radio

    I use in the shed. I hit the on button and, after a month or more, it will utter a few words of radio4 before I realise its been unplugged.

    Better than the DAB which just gurgles like the queen after a no vote unless I get on the ladder and put it on a beam.

    Should that be gargles?

    1. Dave 126 Silver badge

      Re: I've got one in an 8 year old FM radio

      Similarly, don't go poking inside valve amplifiers (especially ones built in Jamaica for pumping out dancehall music very loudly) unless you know what you are doing. The capacitors inside can still hurt you, even weeks or months after the amplifier was last connected to a power supply.

  8. Anonymous Coward
    Anonymous Coward

    Does not compute

    What does this guard against that simple RAID does not? These special SSDs would still be a single point of failure unless paired up, and if paired up the chance of corruption to a particular file is vastly reduced anyway. Let's not forget that if the power goes out, there is already data being lost by virtue of nothing being alive to listen to it. A single server not coming back because of corruption to a critical file shouldn't matter (and if it does, then you have much bigger problems).

    SSDs for low latency, fine. Having your operation fall over when one gets corrupted is inexcusable, no matter how much you spent on them individually IMHO.

    1. Peter Gathercole Silver badge

      Re: Does not compute

      It's because the drive flags back to the RAID adapter that the write is complete before it has actually been committed to disk. The RAID controller will invalidate and delete it's copy in it's battery-backed DRAM, and the only copy that exists for the period until the disk write is complete is in the DRAM in the drive.

      If the RAID set is large enough, you could hope that only one drive's copy is lost, which would allow the data to be reconstructed in RAID modes 5, 6 or 10 (but probably not RAID 1), but I would not want to bet the farm on it

  9. Mike Shepherd

    "...it is becomes increasingly important to learn about the supercapacitors that help prevent data loss"

    It's no more important than learning how my car works or how electricity gets to my house. It may be of interest and no doubt it's a concern for the people who provide those things, but that's what I'm paying them to do.

  10. Humpty McNumpty

    Huh,

    Seriously, the hyped up "supercapacitors" tech articles love to refer to in the vaguest terms these days are actually boring old "Tants" as used on just about every board you might care to pick up from the last 20 years? Never seen on explode that wasn't backwards.

    A typical lead free solder alloy such as SAC305 has a melting point of 217C, to ensure every joint reaches at least this temperature some parts of a PCB can easily hit 250C, most lead free process compatible components can handle up to 260C for a few seconds. The reflow profile for any given product is tuned such that all components reflow while staying within their defined limits for ramp rate and maximum temperature. In the more common convection reflow process this is achieved by selecting an appropriate speed for the conveyor through the oven and the temperature of the hot air forced through nozzles in zones along the length of the oven. The alternative is condesation/vapor phase, whereby the PCB is immersed in a vapor and so heats evenly and gets no hotter than the boiling point of the Galden used.

    You don't hand solder surface mount parts, if you find yourself needing to, your process is shit. More to the point what would be the point in that device being surface mount? Hand soldering a component with a soldering iron infers heating first one lead then the other, this induces stresses (thermal and mechanical) on the component and it could snap in two.

    Automated through hole soldering processes do exist in numerous forms:

    Flow Wave - the whole pcb runs over a wave of liquid solder, only really works if the underside of the PCB has no components on it (OK only true at the pitches involved here).

    Robot - an arm that essentially mimics manual soldering.

    Selective Soldering - The PCB is moved in x and y over a very narrow wave of molten solder soldering only the required areas.

    Pin in Paste - place the though hole part with your surface mount machine, and reflow it on your surface mount process

    Of course as Mike says, none of this matters, or helps you choose a drive.

    On the pictures I have seen from reviews of consumer grade drives, the zoomed in detail of banks of capacitors appear to simply be high capacity MLCC capacitors - probably something like X7R or Y5R. (on the basis of them not being black/yellow/orange and having no polarity markings). These are far inferior to Tantalum based caps in many respects, unstable and they degrade over time.

    1. Anonymous Coward
      Anonymous Coward

      Re: Huh,

      Large tants and electrolytics aren't true supercapacitors, they are just Very Big Capacitors. There are applications (typically <1F) where either can be used.

    2. Alan Brown Silver badge

      Re: Huh,

      "Seriously, the hyped up "supercapacitors" tech articles love to refer to in the vaguest terms these days are actually boring old "Tants" as used on just about every board you might care to pick up from the last 20 years? Never seen on explode that wasn't backwards."

      I've seen non-backwards tants explode but they did have 5 times the rated voltage applied in one case and in the other case the tant was getting a nasty EMF back-kick.

  11. Anonymous Coward
    Anonymous Coward

    This is not new

    I can't believe that this article is being written. Back in 2008, I proposed using phase-change-memory as the code store and write cache in a HDD as an alternative to the architecture proposed in this article. As a result my company began working on a PCM part for this purpose. We filed patents on using a supercap to support PCM during power-loss events since it has internal SRAM buffers that would need to be drained . However, what I learned during the process was that HDD vendors had already placed a piece of NAND memory into the flash drive. So, there was a commodity DRAM and NAND and all that was needed was the energy to do the final data transfer. NVDIMM is a simple extension of this concept which I'm kicking myself for not pioneering duh?

  12. Anonymous Coward
    Anonymous Coward

    SLC cache vs supercapacitor

    some drives are using SLC NAND for a dedicated write cache instead of RAM, which is reserved for a read cache and read-only copy of the block map, to avoid the expense of a supercapactor. That may not be the best idea for a write heavy application like a database, but for consumer use it would be fine.

  13. Alan Brown Silver badge

    Disk buffers

    "The DRAM buffers cannot be set to read only: they are on or they are off."

    Bzzt, WRONG.

    Write buffering can be disabled on spinning HDDs (hdparm -W0), but read caching generally can't be as it would devastate performance. Switching off write buffering is bad enough.

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